Lithium batteries are often utilized in systems where high energy density per volume or weight is desirable. Lithium batteries or cells may be button shaped, cylindrically or prismatically wound, or flat, composed of layers, known as planar cells or planar batteries. In most instances lithium based electrochemical cells are rechargeable, or are referred to as secondary batteries. Lithium electrochemical cells or batteries include a negative electrode or anode, containing a substance capable of intercalating lithium, or lithium metal or an alloy of lithium, as the negative active component. The positive active component in the cathode is usually a chalcogenide of a transition metal and lithium, such as lithium-manganese oxide, lithium-cobalt oxide and similar type of compounds. The non-aqueous electrolyte may be a porous separator impregnated with an organic liquid containing a dissolved lithium salt, or a solid polymer containing a dissociable lithium compound, or composed of particles of solid polymer containing dissociable lithium compounds which particles are intermixed with one of the electrode active compound containing particles. Lithium batteries are usually equipped with current collectors in close proximity of the electrodes, which can be a metallic plate, rod, grating or foil, most frequently made of copper or aluminum, or similar metals or alloys thereof. The packaging of thin plate lithium cells often utilize metallic foils carried by a polymeric laminate, in addition to other polymeric layers designed to protect the rechargeable lithium battery from corrosion and mechanical damage. It is noted that the packaging polymeric laminate carrying metallic foil is an insulator and is usually impermeable to liquids and gases. The lithium batteries under consideration may be composed of a single rechargeable lithium electrochemical cell or several rechargeable lithium cells stacked, folded or interconnected in a known manner to make up a lithium battery.
It is of importance that good contact is maintained between the current collector and the respective electrode so that the energy the lithium battery is capable of delivering is maximized under normal conditions. There are known methods to improve electrical contact between the cell electrode and the internal surface of the current collector. One of such methods is designed to eliminate or reduce oxide formation on the surface of the metallic current collector. In another method a conductive polymeric layer is inserted between the current collector and the electrode. By way of examples, a few patents which are directed to conductive polymeric layers, are discussed below. U.S. Pat. No. 5,262,254 issued to Koksbang et al. teaches an electronically conductive polymeric layer which can protect the metallic collector from attack by the cell electrolyte. U.S. Pat. Nos. 5,441,830 and 5,464,707, issued to Moulton et al. on Aug. 15, 1995 and Nov. 7, 1995, respectively, discuss adhesion-promoting polymeric mixtures bearing fine carbon powder, which are coated on a metallic foil, or on a metallic layer supported by a polymer laminate, and then cured or dried. The electrode paste is deposited on the cured or dried adhesion promoting coating. U.S. Pat. Nos. 5,464,706 and 5,547,782, issued to Dasgupta et al. on Nov. 7, 1995 and Aug. 20, 1996, respectively, disclose conductive polymeric layers loaded with ceramic or carbon particles of certain particle size range, inserted to be in contact with the electrode and the metallic collector surface, with the objective to diminish corrosion. U.S. Pat. No. 5,554,459, issued to Gozdz et al. on Sept. 10, 1996, teaches cleaned collector elements, more specifically a metallic grid, coated with an adherent, carbon loaded, electrically conductive polymer composition. U.S. Pat. No. 5,728,181, issued to Jung et al. on Mar. 17, 1998, discloses a conductive ink composed of a long chained polymer and fine carbon coated on a current collector surface, onto which an electrode layer is subsequently deposited, and the two layers carried by the current collector are then bonded together by radiation or heat curing. However, the resistivity of the above discussed conductive polymeric layers is likely to change with temperature gradually, and temperature induced structural changes within the solid polymeric layers are not known to be reversible.
High temperatures within the battery may be caused by too high overall currents during charging or discharging, or it can arise in the course of normal battery operation, brought about by local irregularities in the interaction between the cell components. Moreover, localized short circuits can lead to high currents within only a small portion of the cell, thereby creating a notable increase of temperature in the neighbourhood of the trouble area or a hot spot in the battery. The locally high temperature can damage the electrolyte or reduce the porosity of the separator, thereby blocking irreversibly the passage of ions, generate harmful gases which may ultimately lead to explosion and fire, or otherwise affect the safe operation of the battery. If some means can be found to diminish the high local current and thus reduce the high local temperature, the battery may be able to continue to operate normally. It is noted that there are several known methods whereby externally installed fuses or switches can stop the battery charging or discharging process, should the current rise beyond a permitted level. Some such fuses are known to operate reversibly. However, such fuses are located in a circuit external of the battery, or are installed in series with the cells, and respond only to the total current drawn through the battery or the cell.
There are known electrically conducting, carbon bearing, blended polymeric compositions, which are capable of exhibiting resistivity changes amounting to several orders of magnitude in a reversible manner in a pre-selected, relatively narrow temperature range. Such compositions are taught, for example, in U.S. Pat. No. 3,793,716, issued to R. Smith-Johannsen on Feb. 26, 1974, and in U.S. Pat. No. 4,237,441, issued to P. van Konynenburg et al. on Dec. 2, 1980. Compositions of such type are utilized in self-limiting electrical heating elements, in heat activated switches comprising electrodes separated by a polymer having positive heat coefficient, in reversible fuses, and in similar devices. A particular application of conductive polymers is described in U.S. Pat. No. 4,957,612, issued to R. F. Stewart et al. on Sept. 18, 1990, wherein a metallic core of an electrode is coated with an electrically conductive polymer, which carries another conductive and electrochemically active component bearing polymer. The resistivities of the conductive polymers are different relative to one another at a given temperature.
There is a need for a thin conductive layer for insertion between the current collector and the electro-active layer in a rechargeable lithium battery, which is capable of reversible resistivity change of several orders of magnitude in response to locally high current flow and attendant local overheating within the battery, thereby being able to render protection from explosion and combustion.